There are 2 levels of difficulty, one for just general fun, and one for competition.
1. once started the game is permanently on, no need to announce it, unless explaining rule to a newbie.
2. each trip starts fresh.
3. whoever sees a purple truck (truck is the front part on an 18 wheeler, not a pickup, delivery van, or U-Haul type vehicle) announces "PURPLE TRUCK" - In our vehicle, decible rating matters, as I am a bit deaf.
4. someone else verifies that it is the color in question, not a similar color (dark blue, dark purple, and black look too similar at a distance).
5. once verified, the person who saw the purple truck gets to choose the next color.
a. sometimes a time limit must be made for choosing the new color.
b. One can specify a color, and exclude the vehicles of that color already present.
*. Example: "Not that red truck"
*. If a color is called on a vehicle in sight, and it is not declaired off limits, it IS fair game
c. Some colors are too rare, and can be voted against by the other players
*. I suggest banning pink... it DOES happen, but rarely enough to hamper the game.
d. colors can be called again, back to back to back
6. Purple Trucks are the trump - a purple truck can be called (and verified) to reset the color.
7. When a new color is declaired, start at point 3, replacing "PURPLE TRUCK" with the new color truck, example "RED TRUCK"
1. above rules apply
2. calling the appropriate color, and having it verified gives the announcer 1 point.
3. miscalling the color removes a point.
a. example: if the appropriate color is black, and someone calls a dark navy blue truck black, they lose the point
b. the miscall must be verified as wrong
4. trumping with a verified purple truck also gains a point.
Be careful, competition actually involves strategy, and most kids pick up the finer points quickly